In semiconductor device manufacturing, projection optical systems are used to transfer integrated circuit (IC) patterns such as large scale integrated (LSI) circuit patterns from a reticle onto a wafer or semiconductor substrate upon which the semiconductor device is to be formed. Because of the difference in the relative sizes of the reticle, which is typically larger than the resulting semiconductor device, projection optical systems must be reduction projection optical systems. A reticle is also known as a mask and the resulting semiconductor device is also known as a die or a semiconductor chip.
Modern integrated circuits are becoming more integrated; that is, more and more functions are being designed into circuits to be manufactured on a single die. However, to maintain or improve the performance and speed of the semiconductor devices being manufactured there is a major effort not to allow the die to grow in size. In order to maintain the same or a reduced die size, the reduction projection optical system for the manufacture of the more complex semiconductor devices must have a wider exposure area, higher resolution, and a higher numerical aperture.
A projection optical system for use in a projection exposure system requires very precise preparation of the glass or quartz materials that are used in the projection optical system. In addition, the projection optical system must be very precisely assembled into the projection exposure system.
The manufacture of current semiconductor integrated circuits typically involves the use of a stepper apparatus or a scanning apparatus. In a stepper apparatus, the reticle or mask is illuminated and the light transmitted through the circuit pattern on the reticle is imaged onto a portion of the photosensitive substrate by the projection optical system. The stepper apparatus then moves the wafer and another portion of the photosensitive substrate is exposed. This process is repeated until the entire photosensitive substrate has been exposed. The photosensitive substrate could be a wafer, or the like, on which a photoresist material has been applied. In a scanning apparatus, the pattern on the reticle is scanned over the surface of the wafer.
The traditional illumination sources utilized in semiconductor manufacturing have been the deuterium lamp and the xenon-mercury arc lamp. The I-line from the xenonmercury arc lamp has a wavelength of 365 nanometers and has been one of the principal illumination sources used in semiconductor manufacturing. In order to attain higher resolution, shorter wavelength optical systems have been developed. These shorter wavelength optical systems use an illumination source such as the KrF excimer laser having a wavelength of 248 nanometers or the ArF excimer having a wavelength of 193 nanometers. In projection optical systems using an excimer laser as an illumination source, the usable lens materials are limited to materials such as quartz and fluorite which can transmit the shorter wavelengths. However, the use of quartz and fluorite materials, which are very expensive, make the cost of these systems very high.
Generally, to accurately transfer a minute pattern from the reticle to the photosensitive substrate by exposure in a projection optical system, the resolving power and depth of focus (DOF) of the projection optical system are important factors. In projection optical systems, the resolving power can generally be improved by increasing the numerical aperture. However, there are trade-offs that must be considered when the numerical aperture is increased. One such tradeoff, for example, is that the depth of focus decreases with an increase in numerical aperture. The relationship between the depth of focus (DOF) and numerical aperture (NA) is DOF=.+-..lambda.(NA).sup.2 where .lambda. is the wavelength of the illuminating light.
Referring to FIG. 1A there is shown an off-axis point A on the reticle 102 with traces A.sub.1 and A.sub.2 representing light rays originating from point A. The ray traces that originate at point A and pass through the pupil defined by the aperture stop AS 35 contribute to form a point image at the point A' on the wafer surface. The ray of light A.sub.1 passing through the center point C on the optical axis A.sub.X in the pupil is called a chief ray or principal ray. The chief ray or principal ray is parallel to the optical axis A.sub.X in the spaces on the object space side and the image space side in the case of a projection optical system in which both sides are telecentric. A ray from an axial object point A.sub.0 on the reticle that passes through the edge of the pupil or aperture stop is called the marginal ray. The sine of the angle .theta. between the marginal ray and the optical axis at the image plane A.sub.0 ' corresponds to the numerical aperture NAw on the wafer side of such a projection optical system and therefore the NA for the system is expressed as NA=Nsin .theta., where N is the refractive index of image space medium that is equal to unity for air. The numerical aperture of such a projection optical system is generally represented as the wafer side value.
To enhance the resolving power, the numerical aperture NA can be increased by making the diameter of the pupil larger. However, before the pupil can be made larger, other parameters of the optical system must be adjusted in order to maintain the performance. As can be appreciated by those of ordinary skill in the optical art, merely increasing the pupil may degrade other optical performance factors such as aberration correction.
Accordingly, there is a need for projection lens systems that have high numerical apertures that are capable of maintaining or increasing the required high level of performance.